Light guiding apparatus

Abstract

A light guide 102 includes at least first 200A and second 200B layers. The respective layers enable light to be guided through the light guide via internal reflections. The apparatus includes in-coupl

Description

TECHNOLOGICAL FIELD Examples of the disclosure relate to light guiding apparatus. Some relate to light guiding apparatus comprising diffractive optics. BACKGROUND Light guiding apparatus comprising diffractive optics can be used in devices such as mediated reality headsets or vehicular displays. Colour uniformity can be problematic in such devices. BRIEF SUMMARY According to various, but not necessarily all, examples of the disclosure there is provided an apparatus comprising: a light guiding member comprising at least a first layer and a second layer wherein the respective layers are arranged to enable beams of light comprising at least light having a first wavelength and light having a second wavelength wherein the second wavelength is shorter than the first wavelength to be guided through the light guiding member via internal reflections; in-coupling diffractive means configured to in-couple one or more beams of light into the first layer of the light guiding member wherein the beams of light comprise at least light having the first wavelength and the second wavelength; out-coupling diffractive means configured to out-couple the one or more beams of light from the light guiding member wherein the in-coupling diffractive means and the out-coupling diffractive means are provided on the first layer of the light guiding member; an interface between the first layer and the second layer wherein the interface is configured to reflect light having the first wavelength and allow light having the second wavelength to pass through; and, wherein at least one of the relative thicknesses of the respective layers and the refractive indices of the respective layers are arranged so that the light having the second wavelength undergoes fewer internal reflections than the light having the first wavelength as the light is guided from the in-coupling diffractive means towards the out-coupling diffractive means. The light guiding member may comprise more than two layers wherein respective interfaces are provided between respective layers and configured so that different interfaces reflect different wavelengths of light and wherein different layers have at least one of different thicknesses and different refractive indices so that different wavelengths of light undergo different numbers of internal reflections as the light is guided from the in-coupling diffractive means towards the out-coupling diffractive means. At least one of the relative thicknesses of the respective layers and the refractive indices of the respective layers may be arranged so as to account for differences in efficiencies of the out-coupling diffractive means for outcoupling the different wavelengths of light. The refractive indices of the respective layers may be arranged so as to increase an internal reflection angle and reduce path length for light of shorter wavelengths. The light guiding member may comprise materials selected to reduce at least one of: path length, absorption, and scattering of light. The first layer may be thinner than the second layer. The layers may progressively increase in thickness. The light guiding member may be substantially planar. The interfaces between respective layers of the light guiding member may be substantially planar. The apparatus may comprise a thin film material at the interfaces between respective layers of the light guiding member. The thin film material may be arranged to reflect light of the first wavelength and reflection angle and allow light of the second wavelength and reflection angle to pass through. The different layers of the light guiding member may comprise different materials. According to various, but not necessarily all, examples of the disclosure there is provided a module, a device, a headset, a vehicle or cab for a vehicle comprising an apparatus as claimed in any preceding claim. While the above examples of the disclosure and optional features are described separately, it is to be understood that their provision in all possible combinations and permutations is contained within the disclosure. It is to be understood that various examples of the disclosure can comprise any or all of the features described in respect of other examples of the disclosure, and vice versa. Also, it is to be appreciated that any one or more or all of the features, in any combination, may be implemented by / comprised in / performable by an apparatus, a method, and / or computer program instructions as desired, and as appropriate. BRIEF DESCRIPTION Some examples will now be described with reference to the accompanying drawings in which: FIGS. 1A to 1E show an apparatus and outcoupling efficiencies for different wavelengths of light; FIG. 2 shows an apparatus according to examples of the disclosure; FIGS. 3A to 3D show an apparatus; FIG. 4 shows brightness levels for an output of an apparatus; FIGS. 5A to 5C show an apparatus; and FIG. 6 shows brightness levels for an output of an apparatus. The figures are not necessarily to scale. Certain features and views of the figures can be shown schematically or exaggerated in scale in the interest of clarity and conciseness. For example, the dimensions of some elements in the figures can be exaggerated relative to other elements to aid explication. Corresponding reference numerals are used in the figures to designate corresponding features. For clarity, all reference numerals are not necessarily displayed in all figures. DETAILED DESCRIPTION Light guiding apparatus comprising diffractive optics can be used in devices such as mediated reality headsets or vehicular displays. Colour uniformity can be problematic in such devices. Figs. 1A to 1E show an example light guiding apparatus 100 and outcoupling efficiencies for different wavelengths of light. Fig. 1A shows an example apparatus 100. The apparatus 100 comprises a light guiding member 102. The light guiding member 102 is arranged to enable light to be guided through the light guiding member 102 via internal reflections. The apparatus 100 also comprises an out-coupling diffractive means 104. The out-coupling diffractive means 104 is configured to out-couple beams of light from the light guiding member 102. A beam of green light 106 is shown passing through the apparatus 100 in Fig. 1A. In the example of Fig. 1A the out-coupling diffractive means 104 is arranged to provide uniform, or substantially uniform, brightness for the output light across the out-coupling diffractive means 104. To achieve this uniform, or substantially uniform, brightness the out-coupling efficiency of the out-coupling diffractive means 104 increases along the path of the beam of the light as it travels through the light guiding member 102. In the example of Fig. 1A the out-coupling efficiency of the out-coupling diffractive means 104 increases from left to right. The power of the remaining light inside the light guiding member 102 reduces as parts of the light are outcoupled. Therefore, to maintain an even level of brightness the efficiency of the out-coupling diffractive means 104 increases. The increase in the efficiency of the out-coupling diffractive means 104 can be achieved by modulating the grating depth or changing any other suitable parameters of the out-coupling diffractive means 104 such as changing the grating fill factor or changing the grating profile. In Fig. 1A the increase in the efficiency of the out-coupling diffractive means 104 is optimized for green light. A beam of green light 106 is shown in Fig. 1A. Fig. 1B shows a plot 120 of the efficiency of the out-coupling diffractive means 104 for green light across the out-coupling diffractive means 104. This plot 120 shows that the efficiency increases from left to right. Fig. 1B also shows a plot 122 of the brightness of the out-coupled green light across the out-coupling diffractive means 104. This plot 122 shows that the brightness is uniform, or substantially uniform. The apparatus 100 is only optimized for a single wavelength of light. In the case of Figs. 1A to 1E the apparatus 100 is optimized for green light. Fig. 1C shows a beam of blue light 130 passing through the apparatus 100 and Fig. 1D shows a beam of red light 140 passing through the apparatus 100. Example wavelengths for the blue, green, and red lights are 460 nm, 525 nm, and 625 nm, respectively. In practical implementations, each light can include a band of wavelengths around these or some other equivalent wavelengths. Fig. 1E shows plots of the outcoupling efficiency and brightness for red and blue light. A first plot 150 shows the efficiency of the out-coupling diffractive means 104 for blue light across the out-coupling diffractive means 104. This plot 150 shows that the efficiency increases from left to right and that the increase is larger than the increase for green light. A second plot 152 shows the brightness of the out-coupled blue light across the out-coupling diffractive means 104. This plot 152 shows that the brightness is not uniform. The brightness of the out-coupled blue light decreases from left to right because the blue light outcouples from the apparatus 100 sooner than the green light. A third plot 156 shows the efficiency of the out-coupling diffractive means 104 for red light across the out-coupling diffractive means 104. This plot 156 shows that the efficiency increases from left to right but that the increase is smaller than the increase for green light. A fourth plot 154 shows the brightness of the out-coupled red light across the out-coupling diffractive means 104. This plot 154 shows that the brightness is not uniform. The brightness of the out-coupled red light increases from left to right because the red light outcouples from the apparatus 100 later than the green light. Examples of the disclosure address the issues of the non-uniformity of the out-coupled light of different wavelengths from such apparatus 100. Fig. 2 shows an apparatus 100 according to examples of the disclosure. The apparatus 100 could be used in a module, a device, a headset such as a mediated reality headset, a vehicle or cab for a vehicle, or could be used for any other suitable purpose. The apparatus 100 is arranged to improve the uniformity of the out-coupled light of different wavelengths. The structure of the apparatus 100 takes into account that different wavelengths of light out-couple from the out-coupling diffractive means 104 with different efficiencies. The apparatus 100 comprises a light guiding member 102 comprising multiple layers 200. The light guiding member 102 comprises at least a first layer 200A and a second layer 200B. In the example of Fig. 2 the light guiding member 102 comprises a first layer 200A, a second layer 200B, and a third layer 200C. The light guiding member 102 can comprise other numbers of layers 200 in other examples. The light guiding member 102 can be substantially planar. The respective layers 200 within the light guiding member 102 can be planar or substantially planar. The respective layers 200 of the light guiding member 102 are arranged to enable light to be guided through the light guiding member 102 via internal reflections. The apparatus 100 also comprises an in-coupling diffractive means 202. The in-coupling diffractive means 202 is configured to in-couple one or more input beams of light into the first layer 200A of the light guiding member 102. The beams of light comprise light of different wavelengths. The beams of light comprise at least light having a first wavelength and light having a second wavelength where the second wavelength is shorter than the first wavelength. For example, the light having the first wavelength could be red light and the light having the second wavelength could be blue light or green light. The apparatus 100 also comprises an out-coupling diffractive means 104. The out-coupling diffractive means 104 is configured to out-couple the one or more beams of light from the light guiding member 102. The in-coupling diffractive means 202 and the out-coupling diffractive means 104 are provided on the first layer 200A of the light guiding member 102. The out-coupling diffractive means 104 has different efficiencies for different wavelengths of light. The diffractive means that are used for the in-coupling diffractive means 202 and the out-coupling diffractive means 104 can comprise any means that can be configured to diffract the input beams of light. The diffractive means can comprise any one or more of a diffractive optical element, diffractive structure, diffraction gratings, holographic gratings, Bragg gratings, rulings, ridges, surface relief diffractive gratings or any suitable optical component or feature having a periodic structure that splits and diffracts light into several beams travelling in different directions. The apparatus 200 also comprises interfaces 204 between the respective layers 200 of the light guiding member 102. The interfaces 204 can be planar or substantially planar. The apparatus comprises a first interface 204A between the first layer 200A and the second layer 200B wherein the first interface 204A is configured to reflect light having the first wavelength and allow light having the second wavelength to pass through. In the example of Fig. 2 the first interface 204A would reflect red light but allow blue and green light to pass through. In examples, where the apparatus 100 has more than two layers 200 the apparatus 100 will comprise more than one interface 204. As shown in Fig. 2A the apparatus 100 comprises two interfaces 204A, 204B. The first interface 204A is between the first layer 200A and the second layer 200B and the second interface 204B is between the second layer 200B and the third layer 200C. The first interface 204A would reflect red light but allow blue and green light to pass through and the second interface 204B would reflect green light but allow blue light to pass through. In some examples the interface 204 can comprise a thin film material between respective layers 200 of the light guiding member 102. The thin film material can be arranged to only allow light with certain internal reflection angle and wavelength to pass through. For example, a thin film material provided at the first interface 204A in the example of Fig. 2A can be arranged to satisfy the condition for total internal reflection (TIR) so as to reflect light of the first wavelength with a first internal reflection angle and allow light of the second wavelength with a second internal reflection angle to pass through. In the example apparatus 100 at least one of the relative thicknesses of the respective layers 200 and the refractive indices of the respective layers 200 are arranged so that the light having the second wavelength undergoes fewer internal reflections than the light having the first wavelength as the light is guided from the in-coupling diffractive means 202 towards the out-coupling diffractive means 104. At least one of the relative thicknesses of the respective layers 200 and the refractive indices of the respective layers 200 are arranged so as to account for differences in efficiencies of the out-coupling diffractive means 104 for outcoupling the different wavelengths of light. For example, the refractive indices of the respective layers 200 can be arranged so as to increase an internal reflection angle and reduce path length for light of shorter wavelengths. The internal reflection angle can be increased relative to a normal axis from the surface of the light guiding member 102. The apparatus 100 can comprise other numbers of layers 200 and interfaces 204 in other examples. In some examples the light guiding member 100 comprises more than two layers 200 and interfaces 204 are provided between respective layers 200. The interfaces 204 can be configured so that different interfaces 204 reflect different wavelengths of light and light with different internal reflection angles. The different layers 200 have different thicknesses and / or different refractive indices so that different wavelengths of light undergo different numbers of internal reflections as the light is guided from the in-coupling diffractive 202 means towards the out-coupling diffractive means 104. In some examples the materials used for the respective layers 200 of the light guiding member 102 can comprise materials selected to reduce at least one of: path length, absorption, and scattering of light. The selection of materials can take into account the relative efficiencies of the out-coupling diffractive means 104 for the different wavelengths of light. Different layers 200 of the light guiding member 102 can comprise different materials where the different materials can provide different path lengths, absorption, and scattering of light for respective wavelengths of light. In some examples, such as the example of Fig. 2, the different layers 200 have different thicknesses. In the example of Fig. 2 the first layer 200A is thinner than the second layer 200B. In this example the second layer 200B is thinner than the third layer 200C. in some examples the layers 200 can progressively increase in thickness from the first layer 200A in the light guiding member 102 to the last layer 200 in the light guiding member 102. Figs. 3A to 3D show beams of light travelling through the example apparatus 100 of Fig. 2. This example apparatus 100 has three layers 200 in the light guiding member 102. The incoupling means 202 and the out-coupling means 104 are provided on the first layer 204A. The first layer 204A has a thickness of 2mm and refractive index of 2.0. The second layer 204B has a thickness of 4mm and refractive index of 2.0. The third layer 204C has a thickness of 10mm and refractive index of 2.0. Other thickness and refractive indices could be used for the respective layers 200 in other examples. A first interface 204A is provided between the first layer 200A and the second layer 200B. The first interface 204A has a thin film layer with a thickness of 5pm and refractive index of 1.55. A second interface 204B is provided between the second layer 200B and the third layer 200C. The second interface 204B has a thin film layer with a thickness of 5pm and refractive index of 1.33. The refractive indices of the thin films are dependent on the refractive indices of the layers 200A-C and other thicknesses and refractive indices could be used for the respective thin films in other examples. In the example of Fig. 3A a beam of red light 300R is shown. The beam of red light 300R is in-coupled to the first layer 200A of the light guiding member 102 by the in-coupling means 202. The interface 204A between the first layer 200A and the second layer 200B is configured to reflect red light with a large internal reflection angle and allow blue light and green light with smaller internal reflection angles to pass through. The beam of red light 300R undergoes total internal reflection at the interface 204A between the first layer 200A and the second layer 200B.The beam of red light 300R therefore stays in the first layer 200A. The distance L-R indicates the total internal reflection length for the beam of red light 300R within the apparatus 100. The total internal reflection length is the distance between consecutive total internal reflections on the first surface of the first layer 200A. In the example of Fig. 3B a beam of green light 300G is shown. The beam of green light 300G is in-coupled to the first layer 200A of the light guiding member 102 by the in-coupling means 202. The interface 204A between the first layer 200A and the second layer 200B is configured to reflect red light with a large internal reflection angle and allow blue light and green light with smaller internal reflection angles to pass through. Therefore, the beam of green light 300G passes from the first layer 200A into the second layer 200B. The interface 204B between the second layer 200B and the third layer 200C is configured to reflect green light with a large internal reflection angle and allow blue light with smaller internal reflection angle to pass through. The beam of green light 300B undergoes total internal reflection at the interface 204B between the second layer 200B and the third layer 200C. The beam of green light 300G therefore stays in the first layer 200A and the second layer 200B. The distance L-G indicates the total internal reflection length for the beam of green light 300G within the apparatus 100. The total internal reflection length for the beam of green light 300G is greater than the total internal reflection length for the beam of red light 300R. This means that there are fewer out-coupling instances of green light than red light in the out-coupling diffractive means 104. In the example of Fig. 3C a beam of blue light 300B is shown. The beam of blue light 300B is in-coupled to the first layer 200A of the light guiding member 102 by the in-coupling means 202. The interface 204A between the first layer 200A and the second layer 200B is configured to reflect red light with large internal reflection angle and allow blue light and green light with smaller internal reflection angles to pass through. Therefore, the beam of blue light 300B passes from the first layer 200A into the second layer 200B. The interface 204B between the second layer 200B and the third layer 200C is configured to reflect green light with a large internal reflection angle and allow blue light with a smaller internal reflection angle to pass through. Therefore, the beam of blue light 300B passes from the second layer 200B into the third layer 200C. The beam of blue light 300B undergoes total internal reflection at the outer surface of the third layer 200C. The beam of blue light 300B undergoes total internal reflection in the light guiding member 102 comprising of the first layer 200A, the second layer 200B, and the third layer 200C. The distance L-B indicates the total internal reflection length for the beam of blue light 300B within the apparatus 100. The total internal reflection length for the beam of blue light 300B is greater than the total internal reflection length for the beam of red light 300R and the total internal reflection length for the beam of green light 300B. This means that there are fewer out-coupling instances of blue light than green light and red light in the out-coupling diffractive means 104. In the example of Fig. 3D a beam of multiple wavelengths of light 300RGB is shown. The respective wavelengths of the light will pass through the light guiding member 102 as shown in Figs. 3A to 3C. The difference in the number of out-coupling instances can account for the difference in out-coupling efficiencies for the different wavelengths of light. This provides for a more uniform colour in the output. Fig. 4 shows plots of the observed brightness of respective wavelengths of light across the out-coupling diffractive means 104 for the apparatus 100 as shown in Figs. 2 and 3A to 3D. The first plot 400 shows the output brightness for red light, the second plot 402 shows the output brightness for green light, and the third plot 404 shows the output brightness for blue light. The respective plots show that the observed brightness is uniform for the different wavelengths of light even though the out-coupling diffractive means 104 is only optimized for one wavelength. Figs 5A to 5C show variations that can be made to the example apparatus 100. In each of the examples of Figs. 5A to 5C the apparatus 100 has three layers 200 in the light guiding member 102 and the in-coupling means 202 and the out-coupling means 104 are provided on the first layer 204A. In the example of Fig. 5A each of the layers 200 has the same refractive index. For example, each of the layers could have a refractive index of 2.0. The same material can be used for each of the layers of the apparatus 100. The thickness of the layers 200 increases progressively through the layers 200 of the light guiding member 102. The second layer 200B is thicker than the first layer 200A and the third layer 200C is thicker than the second layer 200B. The respective thicknesses could be the same as in Figs. 3A to 3D. The respective thicknesses could be 2mm for the first layer 200A, 4mm for the second layer 200B and 10mm for the third layer 200C. Other thicknesses could be used in other examples. In some examples different layers 200 of the light guiding member 102 can have different refractive indices and / or different absorptions. In the example of Fig. 5B the third layer 200C has a different refractive index to the first layer 200A and the second layer 200B. The third layer 200C can have a lower refractive index than the first layer 200A and the second layer 200B. For example, the first layer 200A and the second layer 200B could have a refractive index of 2.0 and the third layer 200C could have a refractive index of 1.6. The same material can be used for first layer 200A and the second layer 200B and a different material can be used for the third layer 200C. The difference in the refractive indices between the second layer 200B and the third layer 200C causes the refraction of the blue light as it passes from the second layer 200B into the third layer 200C. The bending of the light as it is refracted changes the path that the blue light takes through the third layer 200C. This results in an increase in the total internal reflection length compared to an apparatus 100 where the refractive index is the same for all layers such as the apparatus 100 of Fig. 5A. In the example of Fig. 5B the third layer can also have less absorption of the blue light. This can help to increase the brightness of the output of the apparatus 100. In the example of Fig. 5B the respective thicknesses of the layers 200 are the same as in Figs. 3A to 3D and Fig. 5A. The thicknesses could be 2mm for the first layer 200A, 4mm for the second layer 200B and 10mm for the third layer 200C. Other thicknesses could be used in other examples. In some examples the use of different refractive indices and / or absorptions for the different layers 200 of the light guiding member 102 can enable different thickness to be used for the respective layers 200. In the example of Fig. 5C the third layer 200C has a different refractive index to the first layer 200A and the second layer 200B. The third layer 200C can have a lower refractive index than the first layer 200A and the second layer 200B. For example, the first layer 200A and the second layer 200B could have a refractive index of 2.0 and the third layer 200C could have a refractive index of 1.6. The same material can be used for first layer 200A and the second layer 200B and a different material can be used for the third layer 200C. The difference in the refractive indices between the second layer 200B and the third layer 200C increases the total internal reflection length as shown in Fig. 5B. This can enable a thinner layer to be used for the third layer 200C. in this example the thickness of the third layer 200C could be 6.7mm. This provides a total internal reflection length that is similar to the total internal reflection length of the apparatus of Fig. 5A. however, the overall thickness of the light guiding member 102 has been reduced which provides for a lighter apparatus 100. Fig. 6 shows plots of the observed brightness of respective wavelengths of light across the out-coupling diffractive means 104 for the apparatus 100 as shown in Figs. 5A and 5C. The plots on the left-hand side show the observed brightness for the apparatus 100 of Fig. 5A where the same refractive index and absorption is used for all layers 200. The first plot 600 shows the output brightness for red light, the second plot 602 shows the output brightness for green light, and the third plot 604 shows the output brightness for blue light. The respective plots show that the observed brightness is uniform for the different wavelengths of light but that the brightness of the blue light is lower. The plots on the right-hand side show the observed brightness for the apparatus 100 of Fig. 5C where a material with a different refractive index and absorption is used for the third layer 200C. The third layer 200C is also thinner compared to third layer 200C used in Fig. 5A. The fourth plot 606 shows the output brightness for red light, the fifth plot 608 shows the output brightness for green light, and the sixth plot 610 shows the output brightness for blue light. The respective plots show that the observed brightness is uniform for the different wavelengths of light, but that the brightness of the blue light is the same, or substantially the same, as the brightness of other wavelengths. This therefore provides improved colour in the output. The lower output brightness for blue light on plot 604 is mainly caused by two factors. Firstly, travel distance inside the light guiding member 102 for the blue light beams with smaller reflection angles is greater compared to the green and red light beams with larger reflection angles, regardless of the thicknesses of the layers of the light guiding member 102. Secondly, high refractive index glasses typically absorb shorter wavelengths more heavily compared to longer wavelengths. And even though the in-coupling efficiency for blue light can be much higher than the in-coupling efficiencies for red and green light (as the red and green light beams out-couple from the in-coupler more than the blue light beams), the absorption is likely the dominating characteristic defining the lower overall efficiency for the shorter wavelengths. This is especially true with large-size apparatuses 100 used for example as part of head-up display systems. The clear improvement in the output brightness of blue light on plot 610 compared to plot 604 is mainly due to the reduction in the path length inside the light guiding member 102 and due to the lower level of absorption inside the lower refractive index material. In another example, also the second layer 200B material could be replaced with a different material having a smaller refractive index. The second layer 200B could have a different refractive index to the first layer 200A. The second layer 200B could have the same or a different refractive index compared to the third layer 200C. As an example, the first layer 200A could have a refractive index of 2.0, the second layer 200B could have a refractive index of 1.75, and the third layer 200C could have a refractive index of 1.6. While achieving a reduction in the path length while maintaining the total internal reflection length for green light beams, the thickness of the second layer 200B with refractive index of 1.75 could be smaller compared to the original thickness with refractive index of 2.0. This change would improve the overall system efficiency and therefore the output brightness not only for green, but also further for blue. Any suitable materials can be used for the respective components of the apparatus 100. The thin film material that are used in the interfaces 204 can be selected to provide a refractive index that is low enough to support total internal reflection for operational wavelength band and field of view angles. The thickness of the thin film material needs to be substantially larger than the wavelength of the light to reduce any light leakage through frustrated total internal reflection. In addition, the thin film material used for the interface 204 should allow shorter wavelength bands of interest, such as green and blue for the first interface, to pass through without substantial total internal reflection overlap. These requirements can be used to select the refractive indexes and thickness of the layers for the desired optical function. The thin film materials need to be chemically compatible with the surrounding materials. For example, adhesion properties between the thin film materials and adjacent layers 200 should be strong enough so that the layers 200 will not delaminate over the operating conditions. The material used for the layers 200 have to provide for total internal reflection. In addition to this the materials for the layers 200 can be selected so that they will not cause substantial optical absorption or scattering for the light of the wavelengths of interest. In addition to this the materials used for the layers 200 can be selected to provide good mechanical strength. The apparatus 100 works as an exit pupil expander comprising an in-coupling diffractive means 202 configured to in-couple input beams of light and an out-coupling diffractive means 104 configured to expand and out-couple the expanded beams of light. The apparatus 100 may also comprise other diffractive means, such as an expanding diffractive means for expanding the exit pupil of the light beams before passing the light beams towards the out-coupling diffractive means 104, where the exit pupil of the light beams is expanded in a second direction and the beams of light are out-coupled. The efficiency of the expander diffractive means can be modulated in a similar manner as the out-coupling diffractive means 104. In some examples the apparatus 100 can comprise a one dimensional exit pupil expander without the expanding diffractive means can use external optical solutions or slanted mirror surfaces (or prisms) inside the light guide. Beam expansion with refractive optics is possible. In some examples the apparatus 100 could comprise a stack of multiple light guiding members 102, or partially overlapping light guiding members, or adjacent light guiding members for multiplexing colours, focal distance, field of view, exit pupil, or some other features. The out-coupled beams can form a virtual image focused to infinity (using a planar light guiding member), or to a finite distance (using e.g., a spherical light guiding member) or the out-coupled beams can be optically distorted to counter the reflection on a curved combiner element (such as a windshield), or have multiple focal planes (using, for example, a stack of spherical light guiding members). Glare or stray light may be caused by reflections on different surfaces, unwanted diffractions (for example., of higher order) on the diffraction gratings, or some other comparable causes. Means for reducing such effects could be used. Examples include but are not limited to glare shields, anti-reflection coatings of different surfaces, and special diffraction grating solutions. The term ‘comprise’ is used in this document with an inclusive not an exclusive meaning. That is any reference to X comprising Y indicates that X may comprise only one Y or may comprise more than one Y. If it is intended to use ‘comprise’ with an exclusive meaning then it will be made clear in the context by referring to “comprising only one...” or by using “consisting”. In this description, the wording 'connect', 'couple’ and ‘communication’ and their derivatives mean operationally connected / coupled / in communication. It should be appreciated that any number or combination of intervening components can exist (including no intervening components), i.e., so as to provide direct or indirect connection / coupling / communication. Any such intervening components can include hardware and / or software components. As used herein, the term "determine / determining" (and grammatical variants thereof) can include, not least: calculating, computing, processing, deriving, measuring, investigating, identifying, looking up (for example, looking up in a table, a database or another data structure), ascertaining and the like. Also, "determining" can include receiving (for example, receiving information), accessing (for example, accessing data in a memory), obtaining and the like. Also, "determine / determining" can include resolving, selecting, choosing, establishing, and the like. In this description, reference has been made to various examples. The description of features or functions in relation to an example indicates that those features or functions are present in that example. The use of the term ‘example’ or ‘for example’ or ‘can’ or ‘may’ in the text denotes, whether explicitly stated or not, that such features or functions are present in at least the described example, whether described as an example or not, and that they can be, but are not necessarily, present in some of or all other examples. Thus ‘example’, ‘for example’, ‘can’ or ‘may’ refers to a particular instance in a class of examples. A property of the instance can be a property of only that instance or a property of the class or a property of a sub-class of the class that includes some but not all of the instances in the class. It is therefore implicitly disclosed that a feature described with reference to one example but not with reference to another example, can where possible be used in that other example as part of a working combination but does not necessarily have to be used in that other example. Although examples have been described in the preceding paragraphs with reference to various examples, it should be appreciated that modifications to the examples given can be made without departing from the scope of the claims. Features described in the preceding description may be used in combinations other than the combinations explicitly described above. Although functions have been described with reference to certain features, those functions may be performable by other features whether described or not. Although features have been described with reference to certain examples, those features may also be present in other examples whether described or not. The term ‘a’, 'an’ or ‘the’ is used in this document with an inclusive not an exclusive meaning. That is any reference to X comprising a / an / the Y indicates that X may comprise only one Y or may comprise more than one Y unless the context clearly indicates the contrary. If it is intended to use ‘a’, ‘an’ or ‘the’ with an exclusive meaning then it will be made clear in the context. In some circumstances the use of ‘at least one’ or ‘one or more’ may be used to emphasis an inclusive meaning but the absence of these terms should not be taken to infer any exclusive meaning. The presence of a feature (or combination of features) in a claim is a reference to that feature or (combination of features) itself and also to features that achieve substantially the same technical effect (equivalent features). The equivalent features include, for example, features that are variants and achieve substantially the same result in substantially the same way. The equivalent features include, for example, features that perform substantially the same function, in substantially the same way to achieve substantially the same result. In this description, reference has been made to various examples using adjectives or adjectival phrases to describe characteristics of the examples. Such a description of a characteristic in relation to an example indicates that the characteristic is present in some examples exactly as described and is present in other examples substantially as described. The above description describes some examples of the present disclosure however those of ordinary skill in the art will be aware of possible alternative structures and method features which offer equivalent functionality to the specific examples of such structures and features described herein above and which for the sake of brevity and clarity have been omitted from the above description. Nonetheless, the above description should be read as implicitly including reference to such alternative structures and method features which provide equivalent functionality unless such alternative structures or method features are explicitly excluded in the above description of the examples of the present disclosure. Whilst endeavoring in the foregoing specification to draw attention to those features 5 believed to be of importance it should be understood that the Applicant may seek protection via the claims in respect of any patentable feature or combination of features hereinbefore referred to and / or shown in the drawings whether or not emphasis has been placed thereon. l / we claim: 10

Claims

1. An apparatus comprising:a light guiding member comprising at least a first layer and a second layer wherein the respective layers are arranged to enable beams of light comprising at least light having a first wavelength and light having a second wavelength wherein the second wavelength is shorter than the first wavelength to be guided through the light guiding member via internal reflections;in-coupling diffractive means configured to in-couple one or more beams of light into the first layer of the light guiding member wherein the beams of light comprise at least light having the first wavelength and the second wavelength;out-coupling diffractive means configured to out-couple the one or more beams of light from the light guiding member wherein the in-coupling diffractive means and the out-coupling diffractive means are provided on the first layer of the light guiding member;an interface between the first layer and the second layer wherein the interface is configured to reflect light having the first wavelength and allow light having the second wavelength to pass through; and,wherein at least one of the relative thicknesses of the respective layers and the refractive indices of the respective layers are arranged so that the light having the second wavelength undergoes fewer internal reflections than the light having the first wavelength as the light is guided from the in-coupling diffractive means towards the out-coupling diffractive means.

2. An apparatus as claimed in claim 1 wherein the light guiding member comprises more than two layers wherein respective interfaces are provided between respective layers and configured so that different interfaces reflect different wavelengths of light and wherein different layers have at least one of different thicknesses and different refractive indices so that different wavelengths of light undergo different numbers of internal reflections as the light is guided from the in-coupling diffractive means towards the out-coupling diffractive means.

3. An apparatus as claimed in any preceding claim wherein at least one of the relative thicknesses of the respective layers and the refractive indices of the respective layers are arranged so as to account for differences in efficiencies of the out-coupling diffractive means for outcoupling the different wavelengths of light.

4. An apparatus as claimed in any preceding claim wherein the refractive indices of the respective layers are arranged so as to increase an internal reflection angle and reduce path length for light of shorter wavelengths.

5. An apparatus as claimed in any preceding claim wherein the light guiding member comprises materials selected to reduce at least one of: path length, absorption, and scattering of light.

6. An apparatus as claimed in any preceding claim wherein the first layer is thinner than the second layer.

7. An apparatus as claimed in any preceding claim wherein the layers progressively increase in thickness.

8. An apparatus as claimed in any preceding claim wherein the light guiding member is substantially planar.

9. An apparatus as claimed in any preceding claim wherein the interfaces between respective layers of the light guiding member are substantially planar.

10. An apparatus as claimed in any preceding claim comprising a thin film material at the interfaces between respective layers of the light guiding member.

11. An apparatus as claimed in any preceding claim wherein the thin film material is arranged to reflect light of the first wavelength and reflection angle and allow light of the second wavelength and reflection angle to pass through.

12. An apparatus as claimed in any preceding claim wherein the different layers of the light guiding member comprise different materials.

13. A module, a device, a headset, a vehicle or cab for a vehicle comprising an apparatus as claimed in any preceding claim.

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